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    <meta charset="UTF-8">
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    <title>Document</title>
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    <script>
        //undefied类型只有一个值 就是undefined
        //用于声明变量未赋值
        let num
        console.log(num)  //undefied

        //null表示赋值了 内容为空
        //null是尚未创建的对象
        let obj = null
        console.log(obj)

        //区别1
        console.log(undefined + 1)
        console.log(null + 1)
    </script>
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